There are, of course, numerous prior art variable speed alternator electrical systems; one example is the well known automotive alternator having a wound rotor, at least one stator winding, and a rectifier means connected to the stator winding but controlled in part by a connection to the rotor winding so as to produce a 12 volt (nominal) D.C. output.
In some applications, however, it is not possible to use a wound rotor type alternator. In some marine applications, e.g., torpedo systems, it is essential to utilize an alternator having a permanent magnet (hereinafter sometimes "P.M.") type rotor, i.e., no windings on the rotor, the P.M. rotor type alternator having two important advantages over the wound rotor type alternator. First, the P.M. rotor type alternator can supply a much higher power output (for a given alternator size or volume) as compared to a wound rotor type alternator. The second advantage is that the P.M. rotor type alternator output, as controlled by a SCR bridge arrangement, can be easily controlled so as to produce a zero output voltage, this being a critical strategic requirement of certain load systems adapted to be energized from the output of the alternator. To explain, there are certain conditions or strategic situations when it is essential to have the SCR bridge output voltage signal to be zero; failure to have this function accomplished is not acceptable. A wound rotor alternator combined with a conventional SCR bridge will not produce the desired function. However, a P.M. rotor type alternator combined with a SCR bridge will facilitate the desired function.
Thus, the setting for my invention is the necessity (as dictated by various requirements) of having a power supply meeting the following specifications:
(i) a very small but high power and high frequency 3.phi. alternator with a P.M. rotor where the mechanical drive to the rotor would have a substantial variation in angular velocity while the power supply is under load, PA1 (ii) a SCR bridge, and PA1 (iii) a SCR phase control circuit operatively connected to the SCR bridge and the alternator windings in a closed loop system.
However, the technical specifications described above create two significant problems. The first is the very small, high frequency alternator has an output voltage which becomes severely distorted under load; the distortion in fact prevents the use of the state of the art SCR control techniques.
The second problem is related to the fact that P.M. rotor type alternators produce an electrical output, the magntidue of which is a direct function of the rotor speed, i.e., the faster the rotor R.P.M., the greater the output voltage. In fact, under "no-load" conditions, the alternator output voltage and frequency are directly proportional to the rotor speed. The variation in alternator output voltage, as a function of rotor speed, will cause a very undesirable corresponding variation in the output of the SCR bridge if not regulated by closed loop control.
To solve the first problem, a reference circuit was required having an output reference voltage which was (a) synchronized to the alternator frequency, and (b) was substantially free from harmonic distortion which causes jitter/ambiguous operation.
All A.C. power supplies have some degree of harmonic distortion. As indicated, with small, high frequency alternators, under load, this distortion can be very severe; this is shown in FIG. 3. This figure shows deep notches in barely recognizable sine waves. I tried to filter these wave forms with an integrator, however, "flat" spots remained on the otherwise fairly good looking sine waves. These flat spots, when compared with the error signal in the comparator (FIG. 5), produce uncertainties and cause jitter and instability. Successful operation with the flat spots was impossible and I was forced to abandon this approach.
I also considered these two alternatives:
A. Alternator Shaft Mechanical Timing Wheel: A time wheel arrangement with L.E.D. sensors fixed directly to the alternator shaft can provide timing signals. Since we were using a multi-pole machine, we'd need two L.E.D.s--one for pole location and the other to fix the relationship between shaft position and without voltage. Aside from balance problems at very high angular velocities, this approach is difficult-to-impossible to implement in the allocated volume.
B. Separate Rotor/Field Windings: This arrangement would produce nearly perfect sine-waves for timing purposes, but poses the same implementation problems as A above.
To solve the second problem, a special closed-loop regulating system was required.